1. Field of the Invention
This invention relates to an automatic frequency control (AFC) circuit used at a mobile station for digital communication and more particularly to an AFC circuit which causes the oscillation frequency of an oscillator contained in the mobile station to follow the frequency of a signal received from a base station and stabilizes it.
2. Description of the Related Art
Generally, a receiver of a mobile station adopts a superheterodyne system which requires a local oscillator to convert a reception frequency into an intermediate frequency. As the local oscillator, the configuration can be used which includes a high-precision reference oscillator which oscillates at a high frequency and a circuit which converts an oscillation output of the reference oscillator into a predetermined local oscillation frequency. As the reference oscillator, a voltage-controlled and temperature-compensated crystal oscillator (VC-TCXO) can be used. As the means for converting an oscillation output of the reference oscillator into a local oscillation frequency, a circuit such as a multiplying circuit, or a phase locked loop (PLL) synthesizer can be used.
If the oscillation frequency of the local oscillator, i.e. the local oscillation frequency, contains a deviation, the intermediate frequency signal provided by making frequency conversion of a signal received from the base station shifts from the predetermined frequency. If the frequency of the intermediate frequency signal shifts from the predetermined value, the received data cannot accurately be demodulated and the transmission frequency from the mobile station becomes incorrect.
To prevent such trouble, normally an AFC circuit is used at the mobile station to remove or correct the deviation of the local oscillation frequency.
For example, assume that the receiver has first intermediate frequency F.sub.IF1 and second intermediate frequency F.sub.IF2 as intermediate frequencies. Also assume that the control object value of the oscillation frequency of the reference oscillator, i.e. the reference frequency, is F.sub.0 and that the first and second local oscillation frequencies provided by performing steps such as multiplying the value are F.sub.L1 and F.sub.L2 respectively. When the reference frequency does not shift from the object value F.sub.0, the first and second intermediate frequencies provided by frequency conversion, F.sub.IF1 and F.sub.IF2, can be represented by the following expressions using the frequency of the reception signal, i.e. the reception frequency F.sub.R : EQU F.sub.IF1 =F.sub.L1 -F.sub.R EQU F.sub.IF2 =F.sub.L2 -F.sub.IF1 EQU =F.sub.L2 -F.sub.L1 +F.sub.R ( 1)
If the reference frequency contains a deviation, that is, if the reference oscillator oscillates at F.sub.0 (1+.alpha.), the values of the first and second local oscillation frequencies become F.sub.L1 (1+.alpha.) and F.sub.L2 (1+.alpha.) respectively. As a result, the first and second intermediate frequencies also contain a deviation. Assuming that the first and second intermediate frequencies containing a deviation are represented by F.sub.IF1 ' and F.sub.IF2 ', the frequencies F.sub.IF1 ' and F.sub.IF2 ' are represented as follows: ##EQU1## By assigning expression (1), expression (2) can be represented as follows: EQU F.sub.IF1 '=.alpha.F.sub.L1 +F.sub.IF1 EQU F.sub.IF2 '=.alpha.(F.sub.L2 -F.sub.L1)+F.sub.IF2 EQU =.alpha.(F.sub.IF2 -F.sub.R)+F.sub.IF2 ( 3)
As shown in FIG. 6, at the AFC circuit, the second intermediate frequency generally containing a deviation, F.sub.IF2', is counted for gate time G.sub.T =n/{F.sub.0 (1+.alpha.)) where n is an integer, for example, for 100 msec at step S21. The count value D.sub.A is ##EQU2##
In expression (4), c appears only in the second term. Therefore, if the oscillation frequency of the reference oscillator is subjected to feedback control so that the count value D.sub.A becomes the value of the first term EQU n/F.sub.0 .times.F.sub.IF2
the reference frequency can be controlled to the object value F.sub.0. Based on such relationships, the AFC circuit controls the reference frequency. That is, EQU n/F.sub.0 .times.F.sub.IF2 -D.sub.A
is multiplied by predetermined coefficient c to find value D.sub.B at step S22, and the value D.sub.B is used as correction data to control the reference frequency F.sub.0 at step S23.
Thus, the AFC circuit can stabilize the oscillation frequency of a reference oscillator such as a VC-TCXO.
In such a configuration, however, when the electric field input level is low, if fading occurs, an error occurs in the count value D.sub.A, thus it becomes difficult to accurately control the oscillation frequency of the reference oscillator. Even when the electric field input level is high, if a modulation pattern bias or multipath fading occurs, it still becomes difficult to accurately control the oscillation frequency of the reference oscillator.
For example, in the digital cellular communication system under USA specifications, the frequency deviation allowed for a mobile station is a small value of .+-.200 Hz. In consideration of the fact that the transmission frequency of the base station is an 800 MHz band, it is understood that the frequency deviation tolerance is a strict value of .+-.0.25 ppm. On the other hand, in the digital cellular communication system under the USA specifications, the reference frequency stablizing time at hand off is short: Within 130 msec at -90 dBm input and within 250 msec at -103 dBm input.
Therefore, if the time for counting the second intermediate frequency generally containing a deviation, F.sub.IF2 ', is set to, for example, 100 msec counting on a margin for the time 130 msec, a frequency error exceeding the specification of .+-.200 Hz will occur due to fading or any other cause. To eliminate a frequency error caused by fading or modulation pattern bias, the counting time needs only to be prolonged. However, if the time is prolonged, the stabilizing performance at hand off becomes insufficient.
Further, for digital modulation such as .pi./4 shift QPSK (quadriphase phase shift keying), a modulation pattern needs to be random to suitably perform the control described above, that is, in-band constituents of reception frequencies need to be distributed equally with the center frequency as the center. However, if frequency selective fading such as multipath fading or multifrequency fading occurs, the high or low partial frequency constituent is lost with respect to the center frequency of the reception frequencies. For example, if the reception frequency whose high-frequency constituent is lost is converted into an intermediate frequency which is then counted, the count value D.sub.A becomes a value lower than the center frequency. Generally, the delay time caused by multipath fading is 40 .mu.sec at maximum, thus the fading pitch becomes short (25 kHz) and the frequency deviation standard of .+-.200 Hz cannot be satisfied.
A count error is caused by a temporary drop in the input level at fading. Since random noise in nature is counted in the period during which the input level drops, a count error occurs. If a reception signal in analog form is supplied to count processing, a count error (malfunction of digital circuit) may occur due to disorder or incompletion of the waveform of the signal.